You can add custom CSS inside the theme options panel at Advanced → Custom CSS. This is the preferred method of adding CSS, since it will be saved if you update the theme.
React has one main stylesheet style.css in the theme root folder, in which you can make any adjustments. The file is split into sections by numbered comments and has a table of contents at the top to help you find the right section. We recommend adding custom styles in section Custom at the very bottom of the CSS file. Note: when updating the theme you should make a backup of your changes, as the original file will be overwritten.